Semiconductor dice are hermetically sealed in ceramic packages to protect the dice from corrosive elements, principally moisture, in the external environment. Referring to the drawings, FIGS. 1a and 1b illustrate how a base structure 10 of a dual-in-line ceramic package for a semiconductor device is hermetically sealed to a cap structure 12 of the package according to a conventional sealing technique. Structures 10 and 12 are initially separated as shown in FIG. 1a.
Base structure 10 is built up from a rectangular ceramic base 14 having a flat lower surface. An electrically insulating base sealing layer 16 lies on the upper surface of base 14. A rectangular base cavity passes through sealing layer 16 and extends partway into base 14. Although not shown in FIG. 1a, the inner lateral boundary of layer 16 is normally recessed slightly back from the inner wall of base 14 along the cavity. A semiconductor integrated-circuit die 18 is mounted on base 14 within the base cavity.
A group of electrical leads 20, arranged in a digitated pattern, are partly sunk into sealing layer 16 along its upper surface. Each lead 20 extends beyond the outer lateral boundaries of base 14 and layer 16. Leads 20 are bent downward in a symmetrical configuration along the two long outer sides of layer 16. At this point in the sealing operation, the outer ends of leads 20 are usually connected to a lead frame not depicted in the drawings. A corresponding group of electrical bond wires 22 respectively connect leads 20 to bond pads of die 18.
Cap structure 12 is centered on a rectangular ceramic cap 24 having a flat upper surface. Cap 24 has substantially the same length and width as base 14. An electrically insulating cap sealing layer 26 lies on the lower surface of cap 24. A rectangular cap cavity passes through cap sealing layer 26 and extends partway into cap 24. The cap cavity has somewhat greater lateral dimensions than the base cavity but is otherwise in vertical alignment with the base cavity so that the two cavities form a composite die cavity when the package is sealed. As with components 14 and 16 and likewise not indicated in FIG. 1a, the inner lateral boundary of layer 26 is normally recessed slightly back from the inner wall of cap 24 along the cap cavity.
Sealing layers 16 and 26 consist of a glass whose softening point is relatively low. The softening point is the approximate temperature at which the seal glass starts to flow readily. In particular, the seal glass softens at a temperature significantly less than that at which any of components 14, 18, 20, 22, and 24 begin to melt or soften.
To seal structures 10 and 12 together, they are first aligned as indicated in FIG. 1a (except that base structure 10 is usually on top) and brought into contact along sealing layers 16 and 26 and leads 20. Structures 10 and 12 are then heated to a temperature adequate to cause the seal glass to flow but not high enough to cause any significant melting or softening of the other device components. Structures 10 and 12 thereby fuse together along layers 16 and 26 and leads 20.
The final step is to cool the resultant structure down to room temperature. Sealing layers 16 and 26 harden into a unitary glass layer 28. Leads 20 protrude from layer 28 as shown in FIG. 1b. Dashed line 30 in FIG. 1b generally indicates the sealing interface where layers 16 and 26 meet each other.
One difficulty with the foregoing procedure is that air bubbles (or pockets) often form in glass layer 28 along interface 30 during the sealing operation. The air bubbles generally arise in the areas between leads 20. The largest areas between adjacent leads 20 occur in the two rectangular end sections respectively located between the short outer sides of layer 28 and the nearest boundaries of the die cavity. Consequently, the largest bubbles form in the end sections. Area 32 in FIG. 1a indicates a typical location for a large air pocket.
The air bubbles reduce the mechanical strength of the package and help cause cracks to occur in sealing glass 28 during handling, shipping, and thermal cycling. Item 34 in FIG. 1b indicates the location for a typical crack resulting from an air bubble formed at area 32 in FIG. 1a. The cracks lead to leaks--i.e., channels from the die cavity to the outside of the package. As a result of the loss in hermeticity, moisture enters the package and eventually causes device failure.
In Japanese Patent Publication (Kokai) 53-39859, Suduo approaches the leakage problem by creating cap sealing layer 26 in such a way that it is thicker at the areas where the largest air pockets usually form. Suduo's approach is, however, disadvantageous in one of two alternative aspects. If layer 26 is of the normal thickness at the locations for the largest air pockets but is thinner than normal in the remaining area, the integrity of the seal may be significantly reduced in the thinner area. Conversely, if layer 26 is thicker than normal at the areas for the largest bubbles but is of normal thickness elsewhere, some additional fabrication time is needed.